This invention relates to picture tubes more particularly to electron guns having improved potential supplying means.
In general, color picture tubes are provided with an electron gun for generating multi-electron beams within an envelope comprising a panel, a funnel and a neck portion having stem pins. The electron beams are focused on a target after passing through a main electrostatic focusing lens, formed by an electrostatic field between grid electrodes which are contained within the electron gun. The focusing characteristics of this lens is determined by the distance and voltage difference between the grid electrodes, and their respective aperture diameter.
The performance of the electron gun can also be improved by utilizing a long focal length lens which will reduce magnification and spherical aberration. In forming such a long focal length lens, three methods are generally used. The first method requires the controlling of the voltage difference between grid electrodes. With this method, however, the range of voltage control is limited by discharge that occurs between stem pins. Another method consists of using large diameter grid apertures; the use of this method, however, is restricted by the desirability of having narrow neck type picture tubes. This problem is compounded with the use of the larger multi-beam electron guns which further limits the available space within the neck portion. The final method consists of separating the distance between the grid electrodes. As a result, however, the lenses produced are easily affected by undesirable electric fields generated from charges on the inner wall of the neck and supporting rods of the electron gun.
In recent years complex lens gun systems have been developed to overcome the restrictions mentioned above. Such complex lens gun systems can be used to obtain a long focal lens by means of combining a plurality of lenses. For example, the well known bi-potenial lens, the uni-potential lens and the tri-potential lens can be combined with each other. The combination of lenses, however, complicates the electron gun structure and requires various different potentials being applied to the grids. Such potentials, furthermore, must be increased for effective operation. Consequently, as the number of stem pins necessary to supply the various potentials is increased, the distances between the pins will be reduced and the voltage differences therebetween will increase. As a result, undesirable discharge will easily occur between the pins.
These disadvantages can be overcomed by installing a potentiometer within the envelope. Since a potentiometer can supply a plurality of potentials to the grids by dividing the high anode voltage, it will be unnecessary to increase the number of stem pins for applying various different potentials to the grid.
It has been proposed that a plurality of discrete resistors can be arranged in series within the neck portion of the electron gun to function as a potentiometer. With this arrangement, however, there is minimal space between the inner wall of the neck portion and the electron gun to accommodate the resistors. Consequently, a larger diameter neck portion or a smaller electron gun must be utilized to accommodate these resistors. In either case, certain disadvantages will occur. That is, a larger neck will require an increase in the amount of deflecting power, while a smaller electron gun will reduce the quality of the electron beam. Moreover, utilizing smaller resistors within this limited space would necessarily have a reduced power rating. Such results, consequently, would be impractical when used with the high anode voltage.
Another method is directed to forming a potentiometer by applying a thin film resistor by evaporation to the supporting rod of the electron gun. As with the use of smaller resistors mentioned above, a thin film potentiometer would necessarily be impractical due to its low power rating. The application of approximately 25 KV of anode voltage would destroy the thin film resistance layer.